1. Field of the Invention
The present invention relates generally to the field of optical imaging and more particularly to catadioptric optical systems used for microscopic imaging, inspection, and lithography applications.
2. Description of the Related Art
Many optical systems have the ability to inspect or image features on the surface of a specimen, such as inspecting defects on a semiconductor wafer or photomask, or alternately examining a biological specimen on a slide. Microscopes have been used in various imaging situations, including biology, metrology, semiconductor inspection, and other complex inspection applications where high resolution images of small areas and/or features are desired.
Many such imaging systems are currently available, including those disclosed by applicants in various other issued patents and patent applications, including but not limited to U.S. patent application Ser. No. 10/434,374. The objective disclosed in the '374 application is a small catadioptric objective exhibiting beneficial optical qualities. The small size catadioptric optical design of the '374 application exhibits generally good overall performance, especially at high numerical apertures (NAs) over a very broad spectral range in the deep ultra-violet (DUV) region. Chromatic correction becomes increasingly difficult for performance in the DUV region.
In using an inspection design for the applications discussed, it can be beneficial to improve the overall spectral bandwidth of the system. Generally, the spectral bandwidth represents the variance in light wavelengths that may be employed using the design. For example, certain systems may successfully and efficiently employ light in the range of wavelengths from 800 nanometers down to 266 nanometers. These designs tend to have reduced bandwidths at shorter wavelengths, such as 212 nanometers. The previous designs thus include limited or narrow spectral bandwidths in the high end applications discussed.
Based on the optical characteristics desirable in such a design, a high spectral bandwidth can offer improved capabilities when inspecting specimens using light energy in the DUV range. For example, but not by way of limitation, operation within a range from 193 to 266 nm or greater may result in beneficial performance in certain circumstances. Previous designs do not demonstrate this capability
It would therefore be beneficial to provide a system and objective for use in conjunction with standard microscopes and microscopy applications that overcome the foregoing drawbacks present in previously known imaging systems. Further, it would be beneficial to provide an optical inspection system design having improved functionality over devices exhibiting the negative aspects described herein.